This invention relates generally to filter-driers for refrigeration systems and particularly to a molded filter-drier core having increased water capacity while maintaining the physical characteristics of attrition resistance, strength and permeability.
New refrigerants, which have been developed to counter ozone depletion, have considerably higher water solubility, than previous refrigerants and lubricants. For example, for the new refrigerant R-134a, the water solubility is 1200 ppm as compared to 65 ppm for CFC refrigerant R-12 at comparable temperatures.
Similarly, the lubricants such as polyalkylene glycol and the polyester lubricants developed for use with the new refrigerants are hydroscopic compared with the mineral oils used for the CFC refrigerants. For example, the new lubricants commonly have a water solubility of 200 ppm and greater.
In view of the fact the new refrigerants and lubricants have a greater affinity for water it is important to provide refrigerant filter-driers with greater water capacity, without sacrificing other important qualities such as attrition resistance, strength and permeability, acid removal ability, and filtration than have been available for use with the CFC refrigerants.
In filter-driers used for CFC refrigerants, a typical composition by weight is 64% activated alumina, 15% molecular sieve, 2% kaolin and 19% binder. The molecular sieve is by far the greatest contributor to the water removal capacity of such a composition and 15% molecular sieve was more than adequate for the CFC refrigerants and lubricants. However, this molecular sieve content is insufficient to be effective with the new refrigerants such as R134a. One solution is to simply make the filter-drier core larger. However, this results in units which are considerably larger than units used for CFC refrigerants and is commercially unacceptable.
It is possible to increase the molecular sieve content of a composition to an amount considerably in excess of 15%. However, the strength can only be maintained by increasing the density of the composition of the molded core by excessive compression. Unfortunately, the increase in density results in an undesirable reduction in permeability and filtration ability. Reduction in permeability, in turn, results in a reduced flow rate through the filter-drier core which is likewise unacceptable.
The present filter-drier core solves these and other problems in a manner not revealed in the known prior art.